Such a coating preparation per se is known from U.S. Pat. No. 3,350,285. This document discloses a method for coating a metal surface, in particular a thin metal film secured to a plastic substrate used for printed wiring, by means of an immersion process or an electrolytic process, wherein an aqueous solution containing beryllium ions, chromium VI ions and a high molecular weight compound, e.g. polyvinyl alcohol and polyacrylic acid, is employed to provide a rust-proof and corrosion-resistant film on the metal surface.
When metals are being pretreated in preparation for a subsequent finishing process, the metal surfaces like aluminum, steel and galvanized steel usually, after having been degreased and cleaned, have applied to them, first of all, a conversion layer, followed by a so-called "primer" layer. The purpose of applying a conversion layer is, inter alia, to reduce corrosion phenomena of the metal and to increase the moisture resistance of the painted system. The purpose of applying a primer to the conversion layer is to provide a good bonding surface for the finish to be applied afterwards. As a result, corrosion resistance is generally increased further.
The use of a primer, however, entails a number of drawbacks. Firstly, it can be pointed out that any primer has a limited area of application, i.e. a primer of a particular type is not suitable for every type of finish. In addition, the solvent resistance of a primer is limited, which may result in variations in the properties, depending on the type of finish employed. Furthermore, the primer affects the flow, gloss, hardness and colour of the finish. Another drawback of employing a primer is that adhesion between finish and primer depends on the conditions under which these two are employed, such as the layer thickness and the baking temperature. A final disadvantageous factor in employing a primer is formed by the high installation costs and energy costs.
Thus a need for new coating preparations and methods continues to exist, wherein the disadvantages mentioned are reduced or do not occur at all.
A further objective of the invention is to replace the conventional conversion and primer layers by a single coating layer.